Why doesn't my cat like to use the litter tray anymore?

I've got two house cats and noticed they had been going in the cellar for a pee. We've started to make sure the door is always closed so they can't do it anymore but it was rarely open anyway. My youngest cat is 2 & a half and seems to have decided he won't use the litter tray until he's absolutely desperate! He just stands at the cellar door crying, it's a really loud cry and sounds like he's in pain. Eventually he will use the tray but obviously this is not ideal. I always make sure their litter tray is clean and this has only recently started to happen, does anyone have any suggestions what could be wrong?? Thanks.

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He may be suffering from urinary tract problems. Whenever there is a change in behavior, it's essential to have a vet check the cat out to see if there's a medical issue causing the change. If they get a clean bill of health, then they are reacting to something that is stressing them out. the vet or a cat behaviorist can help you figure that out.